The present invention relates to ultrasonic medical imaging and in particular to an improved apparatus and method for making ultrasonic elastography images.
Elastography is a new imaging modality that reveals the stiffness properties of tissue, for example, axial strain, lateral strain, Poisson's Ratio, Young's Modulus, and other common strain and strain-related measurements. The strain measurements may be collected over an area and compiled as a two-dimensional array of data, which may then be mapped to a gray or color scale to form a strain “image”. Analogously, strain measurements may be collected over a volume displayed either three-dimensionally or as a series of stacked two-dimensional images.
In quasi-static elastography, two images of the tissue may be obtained by the ultrasound device in two different states of compression, for example, no compression and a given positive compression. The tissue may be compressed by an external agency such as a probe or the like, or may be compressed by muscular action or the movement of adjacent organs. Strain may be deduced from these two images by computing gradients of the relative local shifts or displacements in the images along the compression axis. Quasi-static elastography is analogous to a physician's palpation of tissue in which the physician determines stiffness by pressing the tissue and detecting the amount the tissue yields under this pressure.
In dynamic elastography, a low-frequency vibration is applied to the tissue and the tissue vibrations are measured, for example, using Doppler detection.
Ultrasonic elastographic images may have considerable image noise arising from a number of sources. In quasi-static elastography, one noise source is so-called “correlation artifacts” which relate to the amount of overlap in the windows of correlation as may be reduced improve resolution.
A number of techniques have been used to minimize these image artifacts, including: global temporal stretching in which the post-compression echo signal is stretched by a known compression factor before estimating axial strain, wavelet de-noising where wavelet techniques are used to smooth the displacement estimates in the wavelet domain without losing edge information, and multi-compression averaging in which multiple readings are taken with smaller compression increments and averaged together.
Typically elastography produces a strain measurement only along the axis of compression. However, lateral strain may be of value both in deducing qualities like Poisson's Ratio and in countering the effects of lateral motion in de-correlating the axial displacement of the tissue. In addition, shear strain images can also be obtained.
Two principal methods have been used to obtain lateral strain. The first described in U.S. Pat. No. 6,270,459 interpolates between successive axial rays or echo signals to provide a basis for horizontal displacement measurement using a correlation technique. A second technique makes the assumption that the tissue being measured is incompressible and simply deduces lateral strain from the axial strain.